The generated voltage of one photovoltaic cell is generally lower than operating voltages of various machinery and appliance and chargers, and thus, in one way of using the photovoltaic cell for operating a machinery and appliance or charging a charger, there is employed a structure in which two or more photovoltaic cells are series-connected (photovoltaic or solar cell module) in a solar energy power generation system. In such a photovoltaic cell module in which two or more photovoltaic cells are series-connected, when a shadow is casted on some cells due to differences in the installation angles of the respective photovoltaic cells and/or a building, etc., causing variations in received light amounts in the respective photovoltaic cells, a cell whose generated electric power is smaller becomes a resistor (an inversely biased diode), which can cause the reduction of the output of the photovoltaic cell module.
More concretely, as well known in this field, the photovoltaic cell has a characteristic that its current changes together with the increase of its generated voltage from 0V, and there is the optimal operating point at which the generated power becomes its maximum (referred to as the maximum electric power point or the optimal operating point.) as illustrated in FIG. 7A. And, in a case of a photovoltaic cell module in which two or more photovoltaic cells are connected in series as mentioned above, it is presumed that the maximum output points of all the photovoltaic cells are substantially coincident with one another so that the current at the common maximum output point will flow through all the series-connected photovoltaic cells. However, actually, the received light amounts in some photovoltaic cells in a photovoltaic cell module can be decreased due to a shadow, etc. and in that case, the power generation characteristic of only the photovoltaic cell of which the received light amount is reduced changes in the direction that the current to the generated voltage is lowered so that its maximum output point will shift. Then, in the circuit configuration where photovoltaic cells are connected in series, the same current will flow through photovoltaic cells having mutually different maximum output points, and thus, when the current is adjusted for the maximum output point of a cell receiving the larger light amount, the cell receiving the smaller light amount (the cell of a small power generation amount) not only generates substantially no power, but also it becomes a resistance against the current, causing the output reduction of the photovoltaic cell module. (Not only a power generation output corresponding to the received light amount in the photovoltaic cell module could not be obtained, but also the output loss would be caused.).
Then, for a device for avoiding such an output reduction due to differences of the received light amounts among the photovoltaic cells, there has been proposed a power generation operating point control circuit device capable of controlling individually an operating point of each of series-connected photovoltaic cells (Non-patent documents 1-3). This power generation operating point control circuit device employs a multistage voltage step up/down chopper circuit for a circuit configuration where two or more photovoltaic cells are connected in series, and controls a generated voltage in each photovoltaic cell so as to make a current at the maximum output point of each photovoltaic cell flow therethrough, thereby enabling all the photovoltaic cells to generates electric power substantially at the respective maximum output point. According to this power generation operating point control circuit device, even a photovoltaic cell of which the received light amount is reduced due to a shadow, etc. can be made to operate at its maximum output point, and therefore, the generated power corresponding to the received light amount of the photovoltaic cell module can be obtained, and also, since a photovoltaic cell of which the received light amount is reduced does not become a inversely biased diode, the output loss will also be reduced.
In this regard, with respect to the structure using a chopper circuit in the operational control of a photovoltaic cell, patent document 1 proposes an example of a circuit configuration which is capable of charging the power generation output of a photovoltaic cell into a charger efficiently by reducing the loss in a switching element.